Wet strength is the ability of paper to retain its integrity upon wetting. This property is critical for tissue, towel, napkin and other consumer products. A typical wet strength product in the marketplace is polyaminoamide-epichlorohydrin resin (PAE resin).
As polyaminoamide-epichlorohydrin resin is an active crosslinker, viscosity can build up and eventually gelling will occur, especially at conditions of prolonged storage time and high temperatures. The pH of final PAE resins is normally adjusted for suitable storage stability, but the higher the solid content, the lower the pH at which the resin must be maintained. However, there is a limit to which the pH value can be lowered since the PAE resin can be hydrolyzed once the pH is below a certain level.
Procedures for making PAE resins are well known in the literature. A polyaminoamide backbone is first prepared by reacting a polyalkylene polyamine and an aliphatic dicarboxylic acid or dicarboxylic acid derivative. A polyaminoamide made from diethylenetriamine and adipic acid or esters of dicarboxylic acid derivatives is most common. The polyaminoamide then reacts with epichlorohydrin in an aqueous solution. The product is diluted and neutralized to the desired solid content and pH range. Nearly all references involve the neutralization of the product with strong mineral acid such as sulfuric acid and hydrochloric acid to a pH below 3.0. The final products are kept at 10-25% concentration to avoid gellation.
A higher solid content is always desirable in the final product due to its lower cost of manufacturing, storage, transportation and application. But due to the decreased stability at higher solid content, wet strength resins are mostly kept at lower concentrations while the pH is adjusted below 3.0 with strong acid such as sulfuric acid and hydrochloric acid to avoid gellation. Two popular wet strength resins on the market are 12.5% KYMENE® 557 series from Hercules Incorporated, Wilmington, Del. and 25% AmRes® 25HP from Georgia-Pacific Resins Inc., Atlanta, Ga. Synthesis of a stable PAE resin of high solid content has remained a challenging task.
A process for stabilizing an aqueous polyamide-epichlorohydrin solution by adding a mixture of a weak acid and a strong acid to the aqueous solution is disclosed in EP0320121. In this process the majority of the acid is the weak acid (preferably formic acid). The preferred mole ratio of weak acid protons to strong acid protons (sulfuric acid) is about 3:1.
U.S. Pat. No. 3,197,427 discloses a process for stabilizing an aqueous polyamide-epichlorihydrin solution in which the pH of the solution is first adjusted to 3-4 with formic acid and is then adjusted to 2-3 with sulfuric acid.